Related Articles

Like other developed nations, the United States relies heavily on fossil fuels. According to the U.S. Department of Energy, less than 10 percent of the country's energy supply is derived from renewable sources (see References 1). The supply of fossil fuels is limited, and burning them creates greenhouse gas emissions; consequently, replacing coal and oil with alternative energy is a high priority (see References 2). Alternative energy sources may be used on a small scale by individual homeowners and businesses, especially in the cases of solar or wind power. They can also be large-scale operations connected to a municipality's existing electric power grid, in which case they supplement or replace conventional power plants.

Solar Power

Solar photovoltaics turn sunlight directly into electric current. The cells that make up solar panels are composed of semiconductor materials that are sandwiched together and specially treated so one layer has an excess of electrons, while the other has an excess of "holes" that electrons can occupy. When sunlight strikes a solar cell, the electrons start to migrate from one layer to the other, creating an electric current that flows through a wire connected to the solar cell. This energy can be stored in battery packs, directed into generators or fed into the power grid. Individual homes and businesses may be powered by a few solar panels; larger arrays like those in the Mojave Desert can operate as stand-alone power utilities. (See References 3 and 4)

Thermal solar uses sunlight to heat fluids, such as molten salts; these fluids transport heat to an exchanger, which turns water to steam. The steam drives a generator-spinning turbine to generate electricity. The generator is connected to the grid, which can convey the electricity to users in nearby cities. Typically, these types of plants use an array of mirrors to focus the sunlight on a "power tower" or similar structure, where the molten salts will best soak up the sun's rays. (See References 2)

Biofuels

Plants --- and some algae and bacteria --- already have a highly efficient system for storing solar energy. Through the complex process of photosynthesis, they use sunlight to synthesize organic compounds from carbon dioxide and water. By burning biomass --- organic material, such as plant matter or animal waste --- or the liquid fuels derived from it, some of the stored solar energy can be extracted and used to power businesses, homes and cars. (See References 2 and 5)

The most common types of biofuels in use today are ethanol and biodiesel. Ethanol is an alcohol. It's typically produced by using yeast to ferment sugars from biomass, such as corn or sugarcane. The yeast cells release ethanol as a byproduct of their metabolism, and the ethanol can be distilled to make it more concentrated. Biodiesel, another common biofuel, is made by combining fats, such as vegetable oils, with alcohols, such as methanol, in the presence of a catalyst. Some researchers are working on producing biofuels from genetically modified algae and bacteria, because they will have lower impacts than ethanol or biodiesel, both of which require a lot of farmland to produce. (See References 2 and 5)

Hydroelectric and Tidal Power

Driven by gravity, streams and rivers surge downhill toward the sea. Humans can take advantage of this natural process by blocking rivers with large concrete dams. Water builds up behind the dam and feeds into an intake; the intake leads into a conduit that conducts the water through the dam to the other side. On its way through the conduit, the water turns a propeller-like turbine connected to a generator. Current from the electric generator feeds into the power grid. (See References 6)

Tidal power also relies on gravity, but the gravitational influence in this case is that of the sun and moon, which cause the tides. Some designs use a barrage or dam to trap water at high tide. At low tide, the water is released through turbines just as in a hydroelectric dam, thereby generating electricity. The world's oldest tidal facility in La Rance, France, uses this design. Other schemes use tidal turbines that look much like windmills. Tidal currents spin these underwater turbines, which are connected to generators; by turning the generators, they produce electricity. (See Resources 2)

Wind Power

Wind turbines take advantage of wind currents; as the moving air masses strike the turbine blades, the blades rotate. The turbine blades are connected to a shaft, and as the shaft turns, it spins a generator that produces electric current. A wind turbine is like the opposite of a fan: A fan uses electricity to spin a motor and move air, whereas a wind turbine uses moving air to spin a generator and produce electricity. The most common kind of wind turbine has a horizontal axis and looks like a giant propeller, although some turbines have a vertical axis and look a little like eggbeaters. Small wind turbines provide up to 100 kilowatts, enough to sustain homes or store energy by recharging batteries; these are sometimes used in remote off-the-grid locations. Larger turbines provide up to several megawatts and are usually grouped together in large wind farms, which supply electricity directly to the grid. (See Resources 1)

About the Author

Based in San Diego, John Brennan has been writing about science and the environment since 2006. His articles have appeared in "Plenty," "San Diego Reader," "Santa Barbara Independent" and "East Bay Monthly." Brennan holds a Bachelor of Science in biology from the University of California, San Diego.